The last frontier where mankind can explore, utilize and eventually colonize is outer space. The deciding factor that will determine how fast and to what extent mankind will advance into and develop this new frontier ultimately depends upon how expensive it is to get there. The reusable ground to orbit shuttle vehicle represents a considerable advance toward developing an economical space transportation system. Before this system was developed, launch vehicles were expendable. Consequently, the cost of transporting passengers and/or bulk cargo to earth orbit with those first generation launch vehicles was enormous. Thus, the development of reusable launch vehicles represented an important step in reducing the cost of transporting payloads to earth orbit.
Although designs are currently being advanced to develop more economical reusable ground to orbit space shuttles, there is one fundamental problem that appears to be insolvable. This problem can be called the "initial mass problem" and is inherent in all space vehicles propelled by chemical rocket engines. This problem can best be described by considering the well known "rocket equation". ##EQU1## In this equation M.sub.1 and M.sub.2 represent a rocket vehicle's total mass before and after burning its engine to achieve a velocity change denoted by .DELTA.V. The engine's exhaust velocity is denoted by u. The ratio of initial mass to final mass represented by M.sub.1 /M.sub.2 is called the "mass ratio". Thus, the amount of fuel M.sub.f required to achieve a velocity change .DELTA.V is given by M.sub.f =M.sub.1 -M.sub.2. The initial mass M.sub.1 can be calculated by multiplying the mass ratio by the vehicle's final mass M.sub.2. Consequently, in order to keep the vehicle's initial mass M.sub.1 (and required fuel load M.sub.f) as low as possible, the mass ratio should be kept as low as possible. For any given .DELTA.V, it follows from equation (1) that the mass ratio can be reduced only by increasing the exhaust velocity u. Unfortunately, chemical rocket engines have a definite upper limit on u that cannot be exceeded because of fundamental thermodynamics. This upper limit is about 4.50 km/sec. Since the minimum .DELTA.V required to reach low earth orbit when aerodynamic drag and gravity losses are taken into consideration is about 9.00 km/sec, the lowest possible mass ratio corresponding to this .DELTA.V for single stage launch vehicles is 7.39.
In order to illustrate the effect of the initial mass problem suppose that the final "dry mass" M.sub.2 of a single stage ground to orbit chemically propelled launch vehicle is 100,000 kg. Since the minimum mass ratio required to achieve Earth orbit is 7.39, the minimum required initial mass M.sub.1 would have to be 739,000 kg, and the required fuel load would be 639,000 kg. Construction experience has shown that the minimum structural mass required for a cryogenic fuel tan is about 10% of the maximum fuel load. Thus, the mass of the fuel tank alone would be about 63,900 kg. This only leaves about 36,100 kg for the remaining structural mass of the vehicle including the payload. These calculations clearly illustrate that it requires a truly enormous launch vehicle, with an enormous fuel load, to orbit even a relatively low mass payload. The initial mass of the U.S. Space Shuttle is over 2,222,000 kg but the maximum payload is only 30,000 kg. Thus, the payload-to-initial launch mass ratio required to achieve earth orbit by the most efficient single stage chemical launch vehicle is about 70. This is the initial mass problem inherent in all rocket vehicles propelled by chemical rocket engines. Of course, staging does alleviate this problem but when completely reusable launch vehicle designs are contemplated, staging does not offer any significant advantage in terms of reducing overall operational costs. (The U.S. Space Shuttle is considered to be a one and one-half stage vehicle.)
Engineers have studied the initial mass problem for many decades. Since the crux of the problem involves the inherently low exhaust velocities of chemical rocket engines, attempts have been made to develop entirely new engines. But the problem is not simply to develop an engine capable of generating higher exhaust velocities. The engines suitable for launch vehicles must be capable of generating very high thrust to weight ratios. For example, the total thrust developed by a launch vehicle that is launched in the conventional vertical attitude must obviously be greater than the total initial weight of the vehicle if it is to climb off the launch pad.
Ion engines have very high exhaust velocities but have absolute upper bounds on their thrust to weight ratios which are small fractions of unity. Thus, they are unsuitable for launch vehicles. Nuclear rocket engines are capable of generating fairly high thrust to weight ratios but the danger of crashing and contaminating a large area of the earth's surface with radioactivity renders such engines impractical for launch vehicles.
Other, more exotic rocket engines, have been proposed for launch vehicles such as laser rocket engines. In these engines, a high power laser beam is directed at the launch vehicle which is captured and focused onto a suitable working fluid. The working fluid is thereby heated to very high temperatures and expelled from the vehicle with exhaust velocities significantly higher than those obtainable with chemical rocket engines. (See "NASA'S Laser Propulsion Project," Astronautics & Aeronautics, Sept. 1982, pp. 66-73 by L. W. Jones and D. R. Keefer.) Unfortunately, the amount of power that must be generated and focused onto a relatively small area in order to accelerate high mass manned vehicles to orbital velocities renders such concepts impractical. (However, these concepts may be useful for launching relatively small unmanned payloads with very high acceleration.)
The scramjet represents another propulsion system designed to circumvent the initial mass problem. Basically, this engine ingests atmospheric air while the vehicle is moving at relatively low altitudes, and sprays it with hydrogen gas which ignites inside the engine forcing the combustion gases to leave the engine faster than the incoming air thereby generating propulsive thrust. Only hydrogen fuel can be used in this engine because of the very short ignition time. Unfortunately, liquefied hydrogen has a very low density which requires fuel tanks five times larger than conventional fuel tanks (which must be thermally insulated since liquefied hydrogen is a very low temperature cryogenic fluid). This results in a high inert structural mass with a corresponding decrease in payload mass. See "Will the Aerospace Plane Work," Technology Review, January 1987, pp. 42-51 by S. W. Korthals-Altres. This problem is compounded by the fact that since the kinetic energy that the vehicle must develop in order to reach orbital velocity is so high (4.times.10.sup.9 Joules/kg) the amount of hydrogen fuel that must be carried by the vehicle to achieve orbital velocity is about 56% of the total initial vehicle mass, even assuming optimal combustion efficiency. See "From Earth To Orbit In A Single Stage," Aerospace America, August 1987, pp. 32-34, by R. A. Jones and C. D. Donaldson. Thus, there are inherent fundamental engineering problems with this propulsion concept that cannot be circumvented even if the scramjet propulsion system can be made to operate as envisioned at orbital velocity (which may be a physical impossibility). The initial mass problem can be reduced somewhat by this propulsion concept, but it will still be there. Moreover, another problem will be introduced involving a severe limitation on payload volume because nearly all of the interior volume of the vehicle will have to be filled with liquefied hydrogen.
Since the scramjet propelled reusable aerospace plane is currently believed to represent the cheapest method for achieving orbit (with an estimated cost of $200/lb compared to $2,000/lb for the Space Shuttle) the prospects for commercial space travel by private individuals in the 21st century appears to be very remote. (For example, it would cost a 200 lb passenger without luggage $40,000 to be transported to orbit on a one-way flight with this vehicle.)
In theory, the most efficient method for propelling payloads into orbit is by means of an electromagnetic accelerator because the cost essentially reduces to the cost of generating an amount of electrical energy equal to the kinetic and potential energy of the total mass that is accelerated to orbit. For example, if the cost of generating electrical energy is 10.cent./KW-hr, this cost is 90.cent./kg or 41.cent./lb for a 200 km high circular orbit. This is 5,000 times cheaper than the U.S. Space Shuttle and about 500 times cheaper than the proposed aerospace plane. Although there are several different types of electromagnetic accelerators (which are also called mass drivers) that have been designed to accelerate bodies to high velocities (i.e., orbital velocities) from the earth's surface, they all have one common characteristic: they all require an evacuated launch tube through which the payload is accelerated. Therefore, unless an evacuated tube of several hundred kilometers is provided, the acceleration of prior art electromagnetic ground to orbit launchers are inherently high, and the mass and physical dimensions of the payload are too small to be of any significant practical value. See "Electromagnetic Launchers," IEEE Transactions On Magnetics, Vol. MAG-16, No. 5, Sept. 1980, pp. 719-721 by H. Kolm et al.
Large objects, such as completely assembled space based interplanetary transfer vehicles (ITVs) with diameters exceeding 25 m and lengths exceeding 100 m would be completely impossible to accelerate to orbit from the earth's surface by any prior art electromagnetic accelerator. In fact, completely assembled payloads with these dimensions could not be accelerated into orbit by any prior art ground to orbit transportation system (or any such system proposed for the future) because they would simply be too large. It is assumed without question and taken for granted that large objects designed for operating in earth orbit will have to be transported there piece by piece, in relatively small sections, and assembled in orbit. The idea of transporting such huge objects as completely assembled ITVs or completely assembled space stations is viewed as so ridiculous in the prior art of astronautics that such possibilities are not even described in science fiction novels.
The size and weight limitations on payloads transportable to earth orbit in current or future launch vehicles is rooted in the basic thrust generating principles used for accelerating payloads to orbit which, for all practical purposes, are believed to be essentially unchangeable because they involve basic laws of physics. However, the discovery of a fundamentally new physical phenomenon or principle could be applied to develop a completely new thrust generating propulsion concept. An example of this type of innovation was the invention of laser heated rocket propulsion where the phenomena of coherent light generation and propagation represented by laser beams was applied to the field of rocket propulsion. See "The Laser Rocket--A Rocket Engine Design Concept For Achieving A High Exhaust Thrust With High Isp," Jet Propulsion Laboratory, TM 393-92, Feb. 18, 1972, by M. Minovitch, and "Laser Rocket," U.S. Pat. No. 3,825,211 filed June 19, 1972, M. Minovitch.
The ground-to-orbit propulsion concept disclosed herein is based upon another such discovery--superconducting materials with high critical temperatures. It will be shown that this propulsion concept will enable payloads to be orbited with mass and physical dimensions far beyond that which were previously believed to be possible. In fact, for all practical purposes, there are no limits on the size and mass of payloads that could be transported to orbit with this propulsion system. Moreover, since this propulsion concept is basically electromagnetic, it also enables the payloads to be transported to orbit with minimum cost. It will be easily capable of transporting to orbit completely assembled reusable space-based interorbital/interplanetary transfer vehicles operating under the generalized theory of rocket propulsion. (See, "Generalized Theory of Rocket Propulsion for Future Space Travel," Journal of Propulsion and Power, Vol. 3, No. 4, July-August 1987, pp. 320-328, by M. Minovitch.)
Providing the technical means for transporting such huge fully assembled vehicles, and other huge objects such as completely assembled manned space stations hundreds of meters in diameter with a mass of thousands of tons, from the earth's surface directly into orbit (previously believed to be a physical impossibility) is a primary object of this invention.